LONG VALLEY OBSERVATORY QUARTERLY REPORTS

COMBINED January – June 2007

Long Valley Observatory

U.S. Geological Survey

Volcano Hazards Program, MS 910

345 Middlefield Rd., Menlo Park, CA 94025

http://lvo.wr.usgs.gov

This report is a preliminary description of unrest in Long Valley caldera and Mono-Inyo Craters region of eastern California. Information contained in this report should be regarded as preliminary and is not be cited for publication without approval by the Scientist in Charge of the Long Valley Observatory. The views and conclusions contained in this document do not necessarily represent the official policies, either express or implied, of the U.S. Government.

LONG VALLEY OBSERVATORY QUARTERLY REPORTS

January-June 2007

CONTENTS

EARTHQUAKES

CALDERA ACTIVITY AND MAMMOTH MOUNTAIN

SIERRA NEVADA ACTIVITY

REGIONAL ACTIVITY

DEFORMATION

SUMMARY OF EDM AND GPS MEASUREMENTS

CONTINUOUS BOREHOLE AND STRAIN MEASUREMENTS

Instrumentation

Highlights

TILT MEASUREMENTS

Instrumentation

Data

MAGNETIC MEASUREMENTS

BACKGROUND

HIGHLIGHTS

CO₂ STUDIES

HYDROLOGIC MONITORING

SUMMARY FOR JANUARY-JUNE 2007

The relative quiescence in Long Valley caldera that began in the spring of 1998 continued through the first half of 2007. The resurgent dome, which essentially stopped inflating in early 1998 and showed minor subsidence (of about 1 cm) through 2001, was followed by gradual inflation through 2002. The deformation pattern since 2003 has been characterized by gradual subsidence that appears to have flattened out in early 2007. The center of the resurgent dome remains some 75 cm higher than prior to the onset of unrest in 1980. Earthquake activity within the immediate confines of the caldera included minor swarms beneath Mammoth Mountain on January 17-26 and March 13 and a swarm beneath the southeast margin of the resurgent dome (2.5 km WSW of Hot Creek) on March 13-15. The largest of these swarm earthquakes was a M=2.1 event on March 15 located ~2.5 km WSW of Hot Creek. Earthquake activity in the Sierra Nevada south of the caldera continued at a higher rate than within the caldera. The largest earthquake in the region was a M=4.6 event on June 12 near Lake Dorothy (1.5 km SSW of Mount Morrison). Aftershocks to this earthquake persisted through the remainder of June and included some 27 M>2 earthquakes. The carbon dioxide flux in the vicinity of Mammoth Mountain remains high but shows evidence of a gradual decline since 1995. Sporadic episodes of geysering in Hot Creek that began in June has continued but at a declining rate.

Up-to-date plots for most of the data summarized here are available on the Long Valley Observatory web pages (http://lvo.wr.usgs.gov).

EARTHQUAKES (D.P. Hill and A.M. Pitt)

Note: Seismic activity in this report uses the automatic computer-generated (Earthworm) solutions rather than the final hand- check (CUSP processing) solutions. The computer-generated epicentral locations and magnitude estimates have become increasingly reliable with time, and they do not suffer from backlogs that can develop in CUSP processing due to an abrupt increase in the rate of earthquake activity elsewhere in northern California.

LONG VALLEY CALDERA AND MAMMOTH MOUNTAIN ACTIVITY:

Low levels of earthquake activity beneath Long Valley caldera and Mammoth Mountain continued through the first six months of 2007. This activity included two earthquake swarms beneath Mammoth Mountain: one that persisted from January 17 through 26 and the other on March 13. Both swarms included rapid-fire sequences of brittle-failure earthquakes (spasmodic bursts) all located at depths less than 5 km. The largest of the earthquakes in these swarms had magnitudes approaching M=2.0 (Figures S1, S3, S8). Just 24 hours after the March 13 Mammoth Mountain swarm, activity shifted to the southeast margin of the resurgent dome with a swarm that began on March 14 and persisted through March 15. This swarm was centered 2.5 km west-southwest of the Hot Creek thermal area at depths between 6 and 7 km. The largest earthquake in this sequence was a M=2.1 earthquake on March 15 (Figures S3, S8).

SIERRA NEVADA ACTIVITY

As has been true since 1999, earthquake activity in the Sierra Nevada block south of the caldera continues at a higher rate than within the caldera with most of the activity concentrated in a band extending from the southern margin of the caldera for some 20 km to the south-southwest (Figures S1-S7). This activity included two M=3.1 earthquakes near Grinnell Lake on April 11 and 13 and a M=4.6 earthquake at 12:23 AM (PDT) on June 12 near Lake Dorothy (1.5 km south-southwest of Mt. Morrison) that produced felt shaking throughout the Mammoth Lakes region. This earthquake was followed by an energetic aftershock sequence that included three M>3 and over 27 M>2 earthquakes through the end of June (Figures S6, S7). This is the largest earthquake in the area since the M=5.6 earthquake (and its M=4.7 aftershock) on May 15, 1999, which had an epicenter beneath McGee Creek approximately 3 km to the southeast.

REGIONAL ACTIVITY

Seismic activity elsewhere in the region included a M=3.0 earthquake on January 10 beneath the Volcanic Tableland near the Pleasant Valley Reservoir in the Owens Gorge (8 km west of Rovana) that was followed by a sequence of small earthquakes that gradually decayed away through the 19^th. A second cluster of small earthquakes occurred in the same area from May 25 through 31 (Figures S1, S5). The largest event in this sequence was a M=2.4 earthquake on May 25. A swarm of earthquakes centered just 2 km northeast of downtown Bishop began on April 1 and persisted through April 4 (Figure S4). The largest, a M=3.0 earthquake at 10:09 PM on the 3^rd, was preceded by a M=2.7 earthquake at 12:26 PM on the 3^rd – both large enough to produce felt shaking in the Bishop area. These earthquakes were centered at depths of 13 to 15 km, respectively.

DEFORMATION

SUMMARY OF EDM AND GPS MEASUREMENTS

John Langbein, Stuart Wilkinson, Mike Lisowski, Eugene Iwatsubo, and Jerry

Svarc

Over the past 7 years, 18 GPS (Global Position System) receivers have been installed within and near the Long Valley Caldera. Of these, 14 were installed by Elliot Endo of the Cascades Volcano Observatory. The locations of the 12 receivers within the caldera are shown in Figure G1. The close correlation of variations in baseline lengths between the EDM measurements that began in 1984 and the GPS measurements, the first of which began in 1999 (Figures G1, G2, G3), has allowed us to discontinue the expensive, labor-intensive EDM measurement in October of 2006. In the future, we will rely entirely on the GPS measurements.

Recent results from the GPS data indicate that the gradual shortening of the baseline lengths by 1 to 3 cm from 2004 through 2006 (consistent with subsidence of the resurgent dome by a comparable amount) may have flattened out over the past six months (Figure G2). Over the long term, the center of the resurgent dome remains some 75 cm higher than prior to the onset of caldera unrest in 1980 (Figure G3). Also see; http://lvo.wr.usgs.gov/monitoring/index.html#deformation

Figure G-1 Map showing 2-color EDM baselines

Figure G2.Line-length changes for the EDM baselines (red crosses) measured from CASA for the period February 12, 1999 through October 3 2007 compared with continuous GPS data for the same lines (black circles). Note that the EDM measurements were discontinued in October 2006.

Figure G3. Line-length changes for the EDM baselines (red crosses) measured from CASA for the period June 1984 through October 3, 2007 compared with continuous GPS data for the same lines (black circles). Note that the EDM measurements were discontinued in October 2006.

CONTINUOUS BOREHOLE STRAIN MEASUREMENTS (Malcolm Johnston, Doug Myren, and Stan Silverman)

Instrumentation

Dilational strain measurements are being recorded continuously at the Devil's Postpile (POP), Motorcross (MX) near the western moat boundary in the south moat, Big Springs (BS) just outside the norhtern caldera boundary, and at Phillips (PLV1), just to the north of the town of Mammoth Lakes. The site locations are shown in Figure D1. The instruments are Sacks-Evertson dilational strain meters and consist of stainless

steel cylinders filled with silicon oil that are cemented in the ground at a depth of about 200m. Changes in volumetric strain in the ground are translated into displacement and voltage by a expansion bellows attached to a linear voltage displacement transducer.

This instrument is described in detail by Sacks et al.(Papers Meteol. Geophys. ,22, 195, 1971).

Data from the strainmeters are transmitted using satellite telemetry every 10 minutes to a host computer in Menlo Park. The data are also transmitted with 24-bit seismic telemetry together with 3-component seismic data to Menlo Park.

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Figure D1. Locations of dilatometers and tiltmeters.

Highlights

The borehole dilatometer data corrected for barometric-pressure show no significant signals during this six-month period (Figure D2). The comparative pore pressure data at the Postpile dilatometer and Big Springs sites are plotted in the second and third panels. The small step at Motocross (bottom panel) in late January was not seen at POPA or BS and may relate to minor slip on the nearby ring fault system.

TILT MEASUREMENTS (Mal Johnston, Roger Bilham, Doug Myren and Stuart Wilkensen)

Instrumentation

Instruments recording crustal tilt in the Long Valley caldera are of two types - 1) a long-base (LB) instrument in which fluid level is measured in fluid reservoirs separated by about 500 m and connected by pipes, which was constructed by Roger Bilham of the University of Colorado, and 2) borehole tiltmeters that measure the position of a bubble trapped under a concave lens. For tiltmeter locations, see Figure D1. Real time plots of the data from these instruments can be viewed at http://quake.wr.usgs.gov/QUAKE/longv.html.

All data are transmitted by satellite to the USGS headquarters in Menlo Park, CA Data samples are taken every 10 minutes. Plots of the changes in tilt as recorded on each of these tiltmeters are shown in Figures T1-T3. Removal of re-zeros, offsets, problems with telemetry and identification of instrument failures is difficult, tedious and time-consuming task. In order to have a relatively up-to-date file of data computer algorithms have been written that accomplish most of these tasks most of the time. Detailed discussion or detailed analysis usually requires hand checking of the data.  Flat sections in the data usually denote a failure in the telemetry. Gaps denote missing data.

All instruments are scaled using tidally generated scale factors.

Figure T1. East-west and north-south components of the long-base tiltmeter for 1 January through June 2006.

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Figure T2. East-west and north-south components for the borehole tiltmeters installed with the Big Springs (BS) and Motocross (MX) dilatometers for January-May 2007.

Figure T3. East-west and north-south tilt components for the shallow borehole tiltmeters for January-May 2007. See Figure D1 for locations.

Highlights

Fig T1 shows the long base data from January 1 to June 1 in 2007. No changes of note are apparent. Data from the short base tiltmeters are shown in Figures T3. Data from the tiltmeters in the deep boreholes at Big Springs and Motorcross are shown in Figure T2.Very little of geophysical interest occurred this period.

MAGNETIC MEASUREMENTS (M.J.S. Johnston, S. Wilkinson, Doug Myren, Y. Sassai, and Y. Tanaka)

Background

Local magnetic fields at 18 sites in the Long Valley Caldera are transmitted via satellite telemetry to Menlo Park every 10 minutes. These and other data provide continuous 'real-time'

monitoring in this region through the low-frequency data system. The location of these sites is shown on Figure M1. Temporal changes in local magnetic field are isolated using

simple differencing techniques.

Figure M1. Locations of differential magnetic field stations within Long Valley caldera. The reference station MGS (not shown) is located along Highway 395 approximately 20 km southeast of the caldera.

Data:

Plots of daily averaged data from the telemetered magnetometer stations in and near the caldera are shown in Figure M2.

Highlights:

Nothing unusual to report for the January-May 2007 period.

CO₂ STUDIES -- Routine measurements indicate no significant changes through the first six months of 2007. We will include a full report for the year with the July-December 2007 report.

HYDROLOGIC MONITORING (Chris Farrar, Jim Howle, and Michelle Sneed: U.S. Geological Survey, Carnelian Bay and Sacramento, CA).

Hydrologic data collected for the USGS Volcanic Hazards Program in this report include ground-water level data from five wells; stream flow, water temperature, and specific conductance from one site on Hot Creek; and estimated thermal water discharge in Hot Creek Gorge (figure H1). Additional data are available on the web at -- http://lvo.wr.usgs.gov/HydroStudies.html

or upon request – contact: Chris Farrar or Jim Howle at Carnelian Bay 530.546.0187.

BACKGROUND

Ground-water levels in wells and the discharge of springs can change in response to strain in the Earth’s crust. The network of five wells and one surface water station provides hydrologic data that contributes to monitoring deformation and other changes caused from magmatic intrusions and earthquakes in Long Valley Caldera.

GROUND-WATER LEVEL MONITORING

Ground-water levels are measured continuously in four wells, LKT, LVEW, SF, and CH-10B (locations in figure H1), using pressure transducers that are either submerged below the water surface or placed above ground and sense back-pressure in a nitrogen-filled tube extending below the water surface. Barometric pressure is also measured at each site using pressure transducers. The data are recorded by on-site data loggers and telemetered on a three-hour transmit-cycle using the GOES satellite and receivers at Menlo Park and Sacramento. All sites are visited monthly to collect data from on-site recorders and to check instrument calibrations.

Data processing is done in the Sacramento Office. Records of barometric pressure are used in combination with the water-level records to determine aquifer properties from the observed water-level response to atmospheric loading and earth tides. The influences of barometric pressure changes and earth tides are removed from the water-level records. The result yields the filtered water-level record that may contain other hydraulic and crustal deformation signals. Filtered data for wells LKT and CH-10B are given in figures H2 and H3.

Figure H2. Hydrographs for well LKT, based on filtered daily mean values. The rise, beginning in mid-2006 from a strong recharge pulse, began to flatten in 2007.

Figure H3. Hydrographs for well CH10B, based on filtered mean daily fluid levels. The large fluid level rise in mid-2006 is due to high recharge from above average precipitation during the winter of 2006. Changes in density caused by increases in fluid temperature (see fig. H5) may also contribute to the rise.

Figure H4. Unfiltered fluid levels in well LVEW in meters above mean sea level.

Figure H5. Maximum temperatures in well CH10B from logging done 1988 to 2007. Temperatures have increased from < 94^o C in 1991 to over 100^o C in 2006 and leveled-off at 101^o C in 2007. The reasons for the temperature change are uncertain but may be in response to deformation of the resurgent dome, opening of fractures caused by local and distant earthquakes, changes in operations of geothermal wells at Casa Diablo, or a combination of these factors.

SURFACE WATER MONITORING

Site HCF is located downstream from the thermal springs in Hot Creek Gorge (figure H1). Stage, water temperature, and specific conductance (figure H6) are recorded every 15-minutes. The data are recorded by an on-site data logger and telemetered every three hours. Specific conductance is a measure of total dissolved ionized constituents. Water at HCF is a mixture of thermal water from springs along Hot Creek and non-thermal water from the Mammoth Creek basin. Changes in specific conductance are related to changes in the mixing ratio of thermal and non-thermal components of stream flow. Water temperatures change in response to ambient temperatures and the mixing ratio.

Figure H6. Discharge, water temperature, and specific conductance at Hot Creek Flume (HCF), based on daily mean data.

THERMAL WATER DISCHARGE ESTIMATE

Estimates of total thermal water discharge (figure H7) are computed from monthly measurements of discharge, and boron and chloride concentrations collected at a non-recording site (HCA) located upstream of the Hot Creek gorge thermal area and at site HCF downstream. The quantity of thermal water discharged to Hot Creek is known to vary in response to seasonal variations in precipitation, snow-melt, earthquakes, and other processes. It is believed that spring discharge may change in response to crustal strain.

Figure H7. Estimated thermal water discharge for springs in Hot Creek Gorge.

Unusual thermal spring activity, including geyser-like fountaining to heights of 2 m above the creek that began in late May 2006, tapered off by the end of 2006. The relatively high (10% higher than the mean) thermal water discharge in the later half of 2006 diminished through the first half of 2007, consistent with visual observations of less vigorous spring activity.